1. Field of the Invention
The present invention relates generally to apparatus and methods for backfilling tunnel liners. More particularly, the present invention relates to a mobile train configured to move longitudinally through a tunnel liner and locally mix and inject foamed cement grout into the annular cavity between the tunnel bore and the liner.
2. Background Art
As tunnelling technology and equipment have improved during the past several decades, the rate at which tunnels can be excavated has significantly increased, as has the distances over which they can be constructed. As a consequence, the distances between tunnel openings or access shafts are frequently very great in modern-day tunnels. For example, it is not uncommon for the distances between access shafts to be on the order of several miles, and between these the tunnel is frequently overlain by a terrain or structure (e.g., an airport) which prohibits drilling from the surface to the tunnel between such access shafts.
In most cases, a tunnel is not complete until a liner has placed along the perimeter of the bored hole. Not only is this usually necessary to prevent the walls of the tunnel bore from eroding or collapsing, but it is also often necessary to protect whatever it is that the tunnel is conducting--water, electrical transmission lines, atomic particle acceleration paths, and so forth--from coming into contact with the surrounding subterranean formation. Such a lining may be installed by a number of ways, but in a typical technique, a tubular tunnel liner is placed within a cylindrical outer bore which forms the tunnel wall; in many cases (particularly in newly constructed tunnels), this outer bore is the excavation formed through the subterranean formation itself, while in other cases (for example, when renovating existing tunnels such as eroded sewer lines), the tubular tunnel liner is pulled or jacked into place inside of a pre-existing outer pipe. In either case, the tunnel liner is necessarily smaller in diameter than the outer bore, so that the necessary clearance is provided between the two to permit installation of the liner, with the result that an annular cavity is formed intermediate the liner and the outer bore. In most cases, this annular cavity must then be filled with material to stabilize the liner within the bore and to prevent material from collapsing from the outer bore and damaging the liner. One generally preferred backfilling technique is to fill this annular cavity with cement grout.
When lining tunnels in the manner described, it has been found economically advantageous to employ relatively thin, lightweight tubular lining material. For example, in renovation of existing cement sewer lines, thin, high density polyethylene pipe liners are often used. While these liners are inexpensive, they are also typically unable to withstand significant external pressures and weight, and this problem can be minimized by using a lightweight foamed cement grout to backfill) such liners. Such foamed cement grouts also possess significant economic advantages in and of themselves: because foamed cement grouts are fluid and non-shrinking, the need for contact grouting is eliminated; also, in a foamed cement grout, a finished foam, comprising a mass or aggregate of bubbles, is mixed in with a cement slurry so that a great many entrained air spaces are created within the grout. Because a relatively large volume of air is thus entrained in the grout, the amount of cement slurry which is needed to backfill or grout a particular size of cavity is significantly reduced. Especially when grouting large tunnel liners which extend over great distances, the cost savings resulting from the reduction in the amount of cement slurry required when using foamed cement grout can be very great.
Although foamed cement grout is thus a highly advantageous material for use in backfilling tunnel liners, this material, once mixed, is often relatively viscous, and frequently compresses and causes friction and backpressure when pumped through conduits. Thus, while the cement slurry is relatively fluid and easy to pump over significant distances, once it is mixed with the finished foam to produce the foamed cement grout, it is no longer feasible to pump the grout over very long distances, whether through a conduit or through the annular cavity between the tunnel bore and liner. This difficulty is seriously aggravated by the fact that, as discussed above, modern day tunnels and shafts often extend for great distances between access points.
Attempts have been made to mix batches of foamed cement grout within a tunnel when grouting the tunnel liner. These attempts have taken the form of transporting dry cement (e.g., in bags) to a very small batch mixer in the tunnel, and then manually dumping the dry cement into the mixer and mixing this with water and foam to form the foamed cement grout. The foamed cement grout can thus be mixed only in small batches, and so this approach is necessarily exceedingly slow and expensive. Furthermore, because the operation is not continuous, it has been found exceedingly difficult to provide adequate quality control of the foamed cement grout which is so produced.
Another approach which has been proposed is to drill small diameter vertical shafts from the surface to the tunnel bore at various points between the access shafts, so as to be able to inject foamed cement grout from the surface into the annular cavity between the tunnel liner and bore at these points. The drilling of such bores is, of course, extremely expensive, and furthermore, as noted above, the tunnel is frequently overlain by terrain which renders the drilling of such bores infeasible.
Thus, there exists a need for an apparatus and method for using foamed cement grout to backfill tunnel liners which avoids the need to pump the foamed cement grout over long distances into the tunnel bore. Furthermore, there is a need for such an apparatus and method for continuously forming and injecting such foamed cement grout, so that high volumes of grout can be placed over long distances quickly, and with a high degree of quality control. The present invention solves the problems cited above, and improved apparatus and methods are hereinafter described for backfilling or grouting a tunnel liner with foamed cement grout.